1. Field of the Invention
Embodiments of the invention generally relate to functional spinal implant assemblies for insertion into an intervertebral space between adjacent vertebrae of a human spine, and reconstruction of the posterior elements to provide stability, flexibility, and proper biomechanical motion. More specifically, embodiments of the invention relate to artificial functional spinal units including an expandable artificial intervertebral implant that can be inserted via a posterior surgical approach and used in conjunction with one or more facet replacement devices to approach an anatomically correct range of motion. Embodiments of the invention may also be inserted via an anterior surgical approach.
2. Description of Related Art
The human spine is a complex mechanical structure including alternating bony vertebrae and fibrocartilaginous discs that are connected by strong ligaments and supported by musculature that extends from the skull to the pelvis and provides axial support to the body. The intervertebral discs provide mechanical cushion between adjacent vertebral segments of the spinal column and generally include three basic components: the nucleus pulposus, the annulus fibrosis, and two vertebral end plates. The end plates are made of thin cartilage overlying a thin layer of hard cortical bone that attaches to the spongy, cancellous bone of the vertebral body. The annulus fibrosis forms the disc's perimeter and is a tough outer ring that binds adjacent vertebrae together. The vertebrae generally include a vertebral foramen bounded by the anterior vertebral body and the neural arch, which consists of two pedicles and two laminae that are united posteriorly. The spinous and transverse processes protrude from the neural arch. The superior and inferior articular facets lie at the root of the transverse process.
The human spine is a highly flexible structure capable of a high degree of curvature and twist in nearly every direction. However, genetic or developmental irregularities, trauma, chronic stress, and degenerative wear can result in spinal pathologies for which surgical intervention may be necessary. In cases of deterioration, disease, or injury, a spinal disc may be removed from a human spine. A disc may become damaged or diseased, reducing intervertebral separation. Reduction of the intervertebral separation may reduce a height of the disc nucleus, which may cause the annulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the annulus begin to buckle and separate, circumferential or radial annular tears may occur. Such disruption to the natural intervertebral separation may produce pain, which may be alleviated by removal of the disc and maintenance of the natural separation distance. In cases of chronic back pain resulting from a degenerated or herniated disc, removal of the disc becomes medically necessary.
In some cases, a damaged disc may be replaced with a disc prosthesis intended to duplicate the function of a natural spinal disc. U.S. Pat. No. 4,863,477 to Monson, which is incorporated herein by reference, discloses a resilient spinal disc prosthesis intended to replace the resilience of a natural human spinal disc. U.S. Pat. No. 5,192,326 to Bao et al., which is incorporated herein by reference, describes a prosthetic nucleus for replacing just the nucleus portion of a human spinal disc. U.S. Patent Application Publication No. 2005/0021144 to Malberg et al., which is incorporated herein by reference, describes an expandable spinal implant.
In other cases, it may be desirable to fuse adjacent vertebrae of a human spine together after removal of a disc. This procedure is generally referred to as “intervertebral fusion” or “interbody fusion.” Intervertebral fusion has been accomplished with a variety of techniques and instruments. It is generally known that the strongest intervertebral fusion is the interbody fusion (between the lumbar bodies), which may be augmented by a posterior or facet fusion. In cases of intervertebral fusion, either structural bone or an interbody fusion cage filled with bone graft material (e.g., morselized bone) is placed within the space where the spinal disc once resided. Multiple cages or bony grafts may be used within that space.
Cages of the prior art have been generally successful in promoting fusion and approximating proper disc height. Cages inserted from the posterior approach, however, are limited in size by the interval between the nerve roots. Therefore, a fusion implant assembly that could be expanded from within the intervertebral space could reduce potential trauma to the nerve roots and yet still allow restoration of disc space height. It should be noted, however, that fusion limits overall flexibility of the spinal column and artificially constrains the natural motion of the patient. This constraint may cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. Thus, an implant assembly that mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution, would be advantageous.
A challenge of instrumenting a disc posteriorly is that a device large enough to contact the end plates and slightly expand the space must be inserted through a limited space. This challenge is often further heightened by the presence of posterior osteophytes, which may cause “fish mouthing” of the posterior end plates and result in very limited access to the disc. A further challenge in degenerative disc spaces is the tendency of the disc space to assume a lenticular shape, which requires a relatively larger implant than often is easily introduced without causing trauma to the nerve roots. The size of rigid devices that may safely be introduced into the disc space is thereby limited.
The anterior approach poses significant challenges as well. Though the surgeon may gain very wide access to the interbody space from the anterior approach, this approach has its own set of complications. The retroperitoneal approach usually requires the assistance of a surgeon skilled in dealing with the visceral contents and the great vessels, and the spine surgeon has extremely limited access to the nerve roots. Complications of the anterior approach that are approach-specific include retrograde ejaculation, ureteral injury, and great vessel injury. Injury to the great vessels may result in massive blood loss, postoperative venous stasis, limb loss, and intraoperative death. The anterior approach is more difficult in patients with significant obesity and may be virtually impossible in the face of previous retroperitoneal surgery.
Despite its difficulties, the anterior approach does allow for the wide exposure needed to place a large device. In accessing the spine anteriorly, one of the major structural ligaments, the anterior longitidinal ligament, must be completely divided. A large amount of anterior annulus must also be removed along with the entire nucleus. Once these structures have been resected, the vertebral bodies are over distracted in order to place the device within the disc and restore disc space height. Failure to adequately tension the posterior annulus and ligaments increases the risk of device failure and migration. Yet in the process of placing these devices, the ligaments are overstretched while the devices are forced into the disc space under tension. This over distraction can damage the ligaments and the nerve roots. The anterior disc replacement devices currently available or in clinical trials may be too large to be placed posteriorly, and may require over distraction during insertion in order to allow the ligaments to hold them in position.